In control of an electric motor, generally, the motor's response with respect to a target value and to an external disturbance are important factors. Namely, it is required that an electric motor respond to instruction values concerning the rotational speed and position of the motor as quickly and stably as possible while external disturbances produce as little effect as possible.
For this purpose, there has been proposed a controller for an electric motor which allows improvement of response by adding a feed-forward control signal, which corresponds to changes in an instruction value and which is obtained by integrating an instruction value, to a feedback control signal corresponding to the amount of deviation between instruction and feedback values and applying the sum as a new control signal. Examples of this type of controller for an electric motor include the motor drive controller disclosed in Japanese Patent Laid-Open Publication No. 107384/1991 and the feed-forward control system for a servo motor disclosed in Japanese Patent Laid Open Publication No. 15911/1991.
However, in the conventional controller for an electric motor, the target value response and external disturbance response cannot be set independently, and if a control system is designed with emphasis on the target value response, the external disturbance response becomes lower, and if a control system is designed with emphasis on the external disturbance response, the target value response becomes lower.
Furthermore, as a feed-forward control signal is obtained by integrating an instruction value, sometimes an excessive feed-forward control signal is generated resulting in an instruction value having an excessive amplitude. As a result, the rotational speed of the motor may be set higher than the allowable speed for the associated torque transfer mechanism or the load machine, or the output torque may be made higher than the allowable torque, causing damage to the load machine.
A detailed description of this effect will hereinafter be given with reference to FIGS. 33 to 43. It should be noted that, in these figures, like reference numerals are assigned to like components.
FIG. 33 shows the general configuration of a conventional rotational speed controller for an electric motor, as disclosed in Japanese Patent Laid-Open Publication No. 91774/1993. In this figure, designated at reference numeral 1 is an electric motor, at 2 a load machine, at 3 a torque transfer mechanism coupled between the electric motor 1 and the load machine 2, at 4 a rotational speed detector for detecting the rotational speed of the electric motor 1, at 340 a differential circuit for differentiating a speed instruction .omega..sub.M * and outputting a feed-forward control signal, at 341 a rotational speed control circuit for comparing the speed instruction .omega..sub.M * to the actual rotational speed .omega..sub.M of the electric motor detected by the rotational speed detector 4, at 342 an adder for adding the output from the rotational speed control circuit 341 to the output from the differential circuit 340 and outputting a torque instruction T.sub.M *, and at 11 a torque control unit for controlling the torque T.sub.M of the electric motor 1 in response to the torque instruction T.sub.M *.
Next a description will be provided of operations conventional rotational speed controller. FIG. 34A is a block diagram showing the rotational speed controller of FIG. 33, wherein G.sub.D is the transmittance of the integration circuit 340 shown in FIG. 33, G.sub.V is the transmittance of the rotational speed control circuit 341, G.sub.T is the transmittance from the torque instruction signal T.sub.M * to the torque T.sub.M of the electric motor 1, and G.sub.M is the transmittance from the torque T.sub.M to the rotational speed .omega..sub.M.
FIG. 34B is a block diagram in which the configuration shown in FIG. 34A is expressed as a transmittance block for the rotational speed instruction .omega..sub.M * and a transmittance block for the external disturbance torque T.sub.L each separated from the other. In this figure, as the only variable parameter in the transmittance block 350 from the external disturbance torque T.sub.L up to the rotational speed .omega..sub.M is G.sub.V, it can be understood that the external disturbance response is determined by only G.sub.V. Also a high target value response can be achieved by arranging the transmittance function block 351 from the rotational speed instruction .omega..sub.M * to the rotational speed .omega..sub.M as a stable polar arrangement with offsetting poles having a low response at a zero point. In this case, the polar arrangement is determined by G.sub.V, and the arrangement of the zero points is determined by G.sub.D and G.sub.V. For this reason, if the external disturbance response is changed by adjusting G.sub.V, the conditions for pole/zero point offsetting in the transmittance block 351 are lost, which may in turn result in overshooting the target value response or may make the response unstable.
Next, what is described above can be verified by means of simulation. For that purpose, at first the transmittance G.sub.T of a torque control unit 11 is obtained. Generally, the torque of an electric motor is controlled by the current in the electric motor, and an embodiment of the torque control unit 11 is shown in FIG. 35. In this figure, designated at reference numeral 330 is a coefficient multiplier for converting a torque instruction T.sub.M * to a current instruction I.sub.M * by multiplying it by the inverse of the torque constant K.sub.T, at 331 a current detector for detecting the current I.sub.M in the electric motor 1, and at 332 a current control circuit for applying to the electric motor 1 a voltage V determined in such a manner that the current in the electric motor 1 follows the current instruction I.sub.M *.
Next, a description will be provided of operations thereof. The transmittance (I.sub.M /I.sub.M *) of a current control system composed of the current detector 331, current control circuit 332, and electric motor 1 is designed so that a first time lag is realized. Furthermore, as the torque T.sub.M is in proportion to the current I.sub.M, the transmittance G.sub.T of the torque control unit 11 is expressed as a first time lag of expression (1) below (See, for instance, "Theory for Servo System and Actual Designing", pp. 80 to 85, and pp. 153 to 155, 1990, Sogo Denshi Shuppan). It should be noted that s is a complex operator, while .omega..sub.CC is the response frequency of the current control system. ##EQU1##
The transmittance G.sub.M from the torque T.sub.M to the rotational speed .omega..sub.M is obtained. Assuming that the sum of inertia of the electric motor 1, that of the load machine 2, and that of the torque transfer mechanism 3 is J, G.sub.M is expressed by expression (2) below. ##EQU2##
Furthermore it is assumed herein that the transmittance G.sub.V of the rotational speed control circuit and the transmittance G.sub.D of the differential circuit are as defined by expressions (3) and (4) below, respectively. ##EQU3##
With the transmittances expressed by expressions (1) to (4) above, if the gains K.sub.V and .alpha..sub.V are set as indicated by expressions (5) and (6) below, respectively, it is possible to realize a speed control system in which the step response does not overshoot because of pole/zero offsetting and the highest response is insured. ##EQU4##
In this case, the transmittance from the speed instruction .omega..sub.M * to the rotational speed .omega..sub.M of the electric motor is as expressed by expression (7) below, while the maximum target value response is .omega..sub.CC /2, namely one-half of the response of the current control system. ##EQU5##
FIG. 36 to FIG. 37 show the results of simulation in a case where a speed instruction .omega..sub.M * with a scale 1 is applied in step fashion and then, in the middle portion of the graph, an external disturbance with a load of scale 40 applied. Herein, J is 1 and .omega..sub.Cc is 2000. FIG. 36 shows the case where K.sub.V is set to 500, K.sub.I to 0, and .alpha..sub.V to 0.5, respectively, according to expressions (5) and (6). FIG. 37 shows a response waveform in the case where K.sub.V is set to 1000 (twice the value in FIG. 36) to improve the external torque response. The latter figure shows that the speed effect due to the external disturbance torque is reduced and the external disturbance response is improved, but overshooting occurs and the target value response becomes lower. Furthermore, FIG. 38 shows the response in a case where the integration gain K.sub.I is set to 10.sup.5 to eliminate a steady-state error due to the external disturbance torque. This figure shows that the steady-state error is eliminated, but overshooting occurs with the target value response deteriorated.
For this reason, it is understood that in the conventional speed controller as shown in FIG. 33 the target value response and external disturbance response cannot be set independently, and that, if the external response is raised, the target value response is deteriorated.
In the conventional speed controller for an electric motor as shown in FIG. 33, the torque T.sub.M generated by the electric motor 1 is restricted by the relation between a mechanical strength of the torque transfer mechanism 3 and that of the load machine 2, and if a speed instruction signal .omega..sub.M * having a large amplitude is inputted, a signal for an excessive torque is outputted from the differential circuit 340, a torque instruction T.sub.M * at a level higher than that allowable in relation to the mechanical strength of the torque transfer mechanism 3 and that of the load machine 2 is generated, and sometimes an excessive torque and vibration are generated, while in some cases the machine may even be severely damaged.
FIG. 39 shows the general configuration of a position controller for an electric motor based on the conventional controller and disclosed in Japanese Patent Laid-Open Publication No. 15911/1991. In FIG. 39, designated at reference numeral 15 is a position/speed detector for detecting the position and rotational speed of an electric motor and outputting an actual speed instruction .omega..sub.M and actual position signal .theta..sub.M, at 370a a first differential circuit for differentiating a positional instruction .theta..sub.M * provided from the outside and outputting a feed-forward control signal, at 371 a position controller for comparing the positional instruction .theta..sub.M * to the actual position .theta..sub.M and outputting a feedback control signal for reducing the error, at 372 an adder for summing the output from the first differential circuit 370a and the output from the position control circuit 371 and outputting a speed instruction signal .omega..sub.M *, at 370b a second differential circuit for differentiating the output from the first differential circuit and outputting a feed-forward control signal, and at 373 a speed control circuit for comparing the speed instruction signal .omega..sub.M * to the actual rotational speed .omega..sub.M and outputting a control signal for reducing the error.
Next, a description will be provided of operations thereof. A detailed description is made with reference to FIG. 40A to FIG. 43. FIG. 40A is a block diagram showing the position controller of FIG. 39, wherein G.sub.D1 is the transmittance of the first differential circuit 370a in FIG. 39, G.sub.P is the transmittance of the position control circuit 371 in the same figure, G.sub.D2 is the transmittance of the second differential circuit 370b in the same figure, G.sub.V is the transmittance of the speed control circuit 373, G.sub.T is the transmittance from the torque instruction signal T.sub.M * to the torque T.sub.M of the electric motor 1, and G.sub.M is the transmittance from the torque T.sub.M to the speed .omega..sub.M.
FIG. 40B is a block diagram expressed with the transmittance block to the positional instruction F.sub.M * in FIG. 40A and the transmittance block to the external disturbance torque T.sub.L, each shown in FIG. 40A and separated herein from each other. In this figure, it is understood that, as variable factors in the transfer block 380 from the external disturbance torque T.sub.L to the position .theta..sub.M are G.sub.P and G.sub.V, the external disturbance response is determined by G.sub.P and G.sub.V. To improve the external disturbance response, the denominator in the transmittance block 380 is required to be made larger, but G.sub.V is included in the s.sup.1 and s.sup.0 terms in the denominator while G.sub.P is included in the S.sup.0 term but is not included in the S.sup.1 term, and for this reason it is understood that it is more effective to make G.sub.V larger to improve the external disturbance response.
An improved target response can be obtained by arranging the transmittance block 381 between the positional instruction .theta..sub.M * and the position .theta..sub.M as a stable polar arrangement and offsetting the poles with a slow response among the poles at a zero point. In that case, the polar arrangement is determined by G.sub.P and G.sub.V, while the arrangement of the zero points is determined by G.sub.D1, G.sub.D2, G.sub.P and G.sub.V. For this reason, for instance, when the external disturbance response is changed by adjusting G.sub.V, the conditions for pole/zero offsetting in the transmittance block 381 are lost and sometimes overshooting may occur or the transmittance may become unstable.
What is described above may be verified through simulation. First, it is assumed that G.sub.T and G.sub.M are expressed by expressions (1) and (2) above. Also, it is assumed herein that G.sub.P and G.sub.V are expressed by expressions (8) to (11) below. EQU G.sub.D1 =.alpha..sub.P s (8) EQU G.sub.D2 =.alpha..sub.V s (9) EQU G.sub.P =K.sub.P ( 10) EQU G.sub.V =K.sub.V ( 11)
In the case where each transmittance is expressed by expressions (1) and (2) and expressions (8) to (11), if the individual gains are set as expressed by the following expressions (12) to (15), respectively, overshooting does not occur because of pole/zero offsetting, and a position control system insuring the highest response can be realized. ##EQU6##
In this case, the transmittance from the positional instruction .theta..sub.M * to the position .theta..sub.M of the electric motor 1 is as expressed by expression (16) below, and the maximum target response is .omega..sub.CC /2, namely, half of the response of the current control system. ##EQU7##
FIG. 41 to FIG. 43 show the result of simulation in a case where a positional instruction .theta..sub.M * with a scale 1 is applied in step form and an external disturbance torque T.sub.M with a scale of 2.times.10.sup.4 is applied starting from the middle portion of the graph. Herein J is 1 and .omega..sub.CC is 2000. FIG. 41 shows the response in a case where K.sub.P is set to 500, K.sub.I to 0, .alpha..sub.P to 0.5, and .alpha..sub.V to 1 according to expressions (12) to (15), respectively, while FIG. 42 shows a response in a case where K.sub.V is set to 2000 (twice that in FIG. 41) to improve the external disturbance response. In FIG. 42, the positional error due to external disturbance torque is smaller and the external response characteristic is improved, but the response shows an initial instability and the target response is deteriorated. FIG. 43 shows the response in a case where an integral gain K.sub.I in the speed control circuit is set to 2.times.10.sup.5 to eliminate steady-state error due to an external disturbance torque. In this case, the steady-state error is eliminated, but overshooting occurs and the target value response is deteriorated. For this reason, in the conventional position controller shown in FIG. 39, the target value response cannot be set independently, and if the external disturbance response is raised, the target value response is deteriorated.
Also in FIG. 39, the rotational speed .omega..sub.M and generated torque T.sub.M in the electric motor 1 are restricted by such factors as the relation between the mechanical strength of the torque transfer mechanism 3 and that of the load machine 2. If a positional instruction signal .theta..sub.M * having a large amplitude is inputted, a signal at an excessive level is outputted from the first differential circuit 370a, and sometimes a speed instruction signal .omega..sub.M * at a level higher than that allowable for the mechanical system or a signal at an excessive level is outputted from the second differential circuit 370b and a torque instruction T.sub.M * at a level higher than that allowable for the mechanical system, and as a result damage may be caused to the mechanical system.
For further background information, reference can be made to Japanese Patent Laid-Open Publication No. 101902/1988 disclosing a control unit, Japanese Patent Laid-Open Publication No. 9404/1987 disclosing a process control unit, Japanese Patent Laid-Open Publication No. 30577/1994 disclosing a speed control unit for an electric motor, Japanese Patent Laid-Open Publication No. 30578/1994 disclosing a position control unit for an electric motor, and Japanese Patent Laid-Open Publication No. 290505/1986 disclosing a process control unit.
As described above, in the conventional controller, there is the problem that the external disturbance response and the target value response cannot be set independently, and especially if it is tried to improve the external disturbance response, the target value response is deteriorated. Also, in a case where an instruction signal having a large amplitude is inputted, as the amplitude of an output from the differential circuit is not restricted, a speed instruction signal or a torque instruction signal at a level higher than that allowable for the machine are generated, which may in turn cause damage to the machine.